Mere mention of the bass trumpet will usually stop a perfectly good discussion in its tracks, if only because so many people otherwise schooled in music have never heard–or even heard of–this odd cross between trumpet and trombone. In fact, most listeners mistake its throaty, polished sound for that of a trombone with valves (itself a relatively rare bird); but because of its compact, more tightly coiled design, the bass trumpet offers more weight and a sharper attack in the lower ranges and an added clarity up top than trombones of traditional design. And for some reason the bass trumpet has a connection with Chi-Town. The instrument gained its most famous champion in Cy Touff, the Chicago hornman who featured it in Woody Herman’s bands; and about the only guy who has recorded on the bass trumpet in recent years is Ryan Schultz, who plays it in several local groups. Schultz’s control of the horn brooks few complaints. If anything, he tends to sound a bit too controlled–the dynamics too evenly modulated–which can flatten out his solos, depriving them of the dynamic emotional boost that would put them over the top. But the improvisations themselves don’t disappoint. They’re smart, splendidly constructed, and devoid of cliches; like a benign but forceful whirlpool, they routinely draw the listener deeper. The repertoire for this engagement leans heavily on tunes by contemporary Chicago jazzmen, and on Schultz’s own provocative and infectious compositions. His quartet will alternate sets with the combo led by trumpeter Orbert Davis, and I doubt anyone would complain if the two leaders lock horns at least once during the night. Saturday, 11 PM and 1 AM, Bop Shop, 1807 W. Division; 235-3232.